Battery-operated vehicles used in mining, for example coal hauling scoops, commonly use one or more series wound DC motors for traction and a compound wound DC motor to drive a hydraulic pump. In such vehicles, the traction motor(s) may not be engaged unless the pump is running at full speed.
Prior to 1980, most traction and pump motor controls for battery-powered vehicles consisted of combinations of resistors and contactors. In the late 1970's, solid-state controllers using SCR's (note that "SCR" and other terms of art are defined following the Detailed Description of the invention), MOSFETS's and BJT's became available but most of these were unreliable, complex and insufficiently powerful for many mining applications. In the early 1990's, more reliable traction motor controllers using newly developed IGBT's became available. Most of these controllers use a single IGBT-based current controller with reversing mechanical contactors to control conventional 4 terminal series wound DC traction motors. More recently, a contactorless reversible IGBT-based motor controller has been produced, using a specially made, dual field series wound "three terminal" DC motor.
U.S. Pat. No. 3,727,118, issued on Apr. 10, 1973, to Makino et al. describes a contactorless reversible controller comprising a load driving circuit including a pair of push-pull circuits formed of bipolar transistors for a three-terminal DC motor. Makino ct al. requires a specially manufactured, three-terminal DC motor, and is inoperable in connection with conventional four-terminal DC traction motors.
U.S. Pat. No. 4,196,377, issued on Apr. 1, 1980 to Trevor C. Boxer, describes an electric vehicle traction motor control circuit including separate armature and field current controls. Boxer uses a speed transducer for motor RPM feedback, unlike some embodiments of the present invention.
U.S. Pat. No. 4,385,266, issued on May 24, 1983, and U.S. Pat. No. 4,406,979, issued on Sep. 27, 1983, both to Sloan, describe a contactorless reversible controller for a three terminal DC motor comprising two thyristor-based static switching devices operated to simultaneously energize their associated fields so as to reduce the next flux in the motor and further increase the motor speed. Like Makino et al., Sloan requires a specially manufactured, three-terminal DC motor, and is inoperable in connection with conventional four-terminal DC traction motors.
U.S. Pat. No. 5,070,283, issued on Dec. 3, 1991 to Avitan, describes a system for controlling separately excited shunt-wound DC motors where control is achieved through microprocessor-based independent PWM control of a chopper (armature) and an H-bridge (field). A contactor is used to by-pass the armature transistor chopper under certain conditions. Avitan is thus reliant upon contactors and does not suggest the contactorless multi-motor controller according to the claimed invention.
The electromechanical contactors required in the prior art are bulky, and are high maintenance items, negating, to a certain degree, the improved reliability gained by the use of the IGBT-based controller. Present solid-state traction motor controllers, associated contactors, and pump starters are supplied in component form and the original equipment manufacture (OEM) or rebuilder of the battery-powered equipment must then mount and wire these components inside the vehicle's enclosure. This is a costly and time-consuming task, and requires the services of an experienced, skilled technician. Moreover, where these components are mounted on-board a battery-operated railed vehicle such as a mining portal bus or locomotive, the components are exposed and can easily be damaged. If the controller fails in use, it is difficult to trouble-shoot, and requires the services of a skilled technician, idling the vehicle until one or more of the defective components are identified and replaced.
The controllers for the three-terminal series wound DC motors referred to in the prior art require motors which are expensive, non-standard, and not readily available.
Controllers requiring speed transducers are not suitable for mining application because they are fragile, require non-conventional motors, and must have prior safety agency approval before use in underground mining.
Existing embodiments of traction motor controllers and pump starters using separately mounted components use up considerable space, which is a significant disadvantage when these components must be housed inside the limited confines of explosion proof enclosures, such as is required for battery-powered vehicles operating near the face of a coal mine.
Moreover, many of the prior art components, most notably the contactors and resistors, are too large, and generate too much heat to allow fitting inside a single, portable compact enclosure which is easy to handle and change out.